We present a crossed molecular beam study of the reaction of O(P-3) with alkanes, using velocity map imaging of the alkyl radical product via single photon ionization at 157 nm. We obtained the differential cross sections and translational energy distributions for reaction of O(P-3) with cyclohexane, n-butane and isobutane at a range of well-defined collision energies from 4.7 to 14.8 kcal/mol. The product alkyl radicals are largely backscattered with respect to the alkane beam at all collision energies, but the scattering distribution is clearly broadened with increasing collision energy. This is consistent with a picture of direct rebound dynamics but inconsistent with a recent result for the vibrationally excited OH component [H. Tsurumaki , J. Chem Phys. 112, 8338 (2000)], and we explore the source of this inconsistency. More surprising is the large fraction of the available energy partitioned into internal degrees of freedom of the alkyl radical, showing that the simple triatomic picture of the reaction is inadequate to account for all of the observations. To explain the observed dynamics we propose a modification of the triatomic model in which the exoergicity is adjusted to reflect "vertical" rather than "adiabatic" H abstraction energetics, and this picture is supported by ab initio calculations and by detailed consideration of the translational energy release patterns.